1. Field of the Invention
The invention generally relates to the concentration or beneficiation of minerals and other particulate matter by froth flotation, and more particularly relates to a method and apparatus for concentration or beneficiation of particulate matter separated from undesired waste by means of froth flotation.
2. Brief Description of Related Technology
Commercially valuable substances, such as coal and minerals, are commonly found in nature mixed with relatively large quantities or prohibitive quantities of unwanted substances, As a consequence, it is usually necessary to beneficiate or clean ores to concentrate a desired substance or, put another way, reduce the content of an unwanted substance. Similarly, recycling processes, such as de-inking of paper fibers, involve the separation of a desired substance (paper fibers) from an unwanted substance (ink).
Mixtures of finely-divided product particles and finely-divided waste particles can be separated and concentrates obtained therefrom by froth flotation techniques. Generally, froth flotation involves conditioning a liquid, commonly aqueous, pulp (or slurry) of the mixture of product and waste particles with one or more frothing agents and optional reagents, and aerating the pulp. The conditioned pulp is aerated by introducing into the pulp a plurality of air bubbles which tend to become attached to either the product particles or the waste particles, thereby causing these particles to rise and generate a float fraction of froth on the surface of a non-float fraction of pulp. The difference in density between air bubbles and water provides buoyancy that preferentially lifts hydrophobic solid particles to the surface. In known processes, the float fraction overflows or is skimmed from the flotation apparatus.
Froth flotation is often used to separate solids of similar densities and sizes, which factors prevent other types of separations based on gravity that might otherwise be employed. It is especially useful for particle sizes below about 100 μm (about 150 mesh), which are typically too small for gravity separation using jigging and tabling. The lower-size limit for flotation separation is typically about 35 μm (about 400 mesh). At smaller particle sizes, it becomes difficult to take advantage of surface-property differences to induce selective hydrophobicity. On the other hand, particles greater than about 200 μm (about 65 mesh) tend to be readily sheared from bubble surfaces by collision with other particles or vessel walls.
Today, at least 100 different minerals, including almost all of the world's copper, lead, zinc, nickel, silver, molybdenum, manganese, chromium, cobalt, tungsten, and titanium, are processed using froth flotation. Another major usage of froth flotation is by the coal industry for desulfurization and the recovery of fine coal, once discarded as waste. Since the 1950's, flotation has also been applied in many non-mineral industries including sewage treatment; water purification; paper de-inking; and chemical, plastics, and food processing.
In conventional subaeration cells, the pulp ordinarily is aerated by means of a mechanical impeller-type agitator and aerator which extends down into the body of pulp and which disperses minute bubbles of air throughout the body of pulp by vigorous mechanical agitation of the pulp.
In conventional froth-flotation columns, air for aeration is introduced directly into a relatively quiescent body of pulp by means of an air diffuser or sparger which is immersed in or in direct contact with the pulp, or by introduction of pre-aerated water, e.g. from below a flotation compartment.
Generally, subaeration cells have a relatively higher throughput than froth-flotation columns, but froth-flotation columns can provide better separation between desired and undesired components. As a consequence, when both high throughput and good separation are desired, subaeration cells typically are used in series and froth-flotation columns are used in parallel. In some cases, the flotation operations are conducted in stages wherein the concentrate obtained from the float fraction in one stage can comprise a different substance from the concentrate obtained from the float fraction in another stage.
Typical undesired impurities in coal include pyrite, sulfur, and other ash-forming mineral matter. Pyrite in many U.S. coals occurs in large quantities as fine-grained matter varying in size between 20 microns (μm) and 32 μm. In some coals, such as is available in Illinois, a significant part of the pyrite is less than 20 μm. To make use of these types of coals more fully, a coal cleaning method capable of processing very finely ground coal in which most of the pyrite particles have been liberated must be used. Similarly, reduction in or removal of ash-forming matter can improve marketability and heat content of cleaned coals, because ash is incombustible and has been linked to poor heat exchange and reduced boiler performance.
In addition, because every coal mine and preparation plant produces fines in the course of extracting and processing coal, failure to recover coal from fines increases the proportion of produced coal that is discharged into the environment (e.g., into tailing ponds) which results not only in a loss of potential revenue but also in an environmental impact.
The separation of fine particles by froth flotation techniques presents particular obstacles which are only overcome with great difficulty and cost by known techniques, such as use of multiple machines in series or parallel, and known techniques still have limitations in the degree of separation which can be achieved.
Thus, it is a continuous goal in the industry to have methods and apparatus which improve the separation of desired particulate matter from undesired particle matter.